The invention relates to a method for detecting an overload situation in a hand-guided power tool equipped with an electric motor, and to a corresponding device.
Methods for detecting an overload situation in a hand-guided power tool equipped with an electric motor are known from the prior art. The term “hand-guided power tool” is understood to mean, for example, screwdrivers, drill/drivers, circular saws, jigsaws, power planers, rotary hammers, or impact drills. In addition to hand-guided power tools that are driven by line current, there are also hand-guided power tools powered by rechargeable battery packs on the market. The aspects discussed below are intentionally focused primarily on hand-guided power tools powered by rechargeable battery packs, but the explanations naturally also apply to hand-guided power tools powered by line current. In power tools, situations can arise in which so much strain is placed on the motor that it comes to a standstill. When at a standstill, though, the electric motor continues to present only a very low ohmic resistance in relation to the energy source, resulting in very powerful flows of current. A motor at a standstill acts on a rechargeable battery pack in a fashion similar to a short circuit. It should also be noted that when the motor is at a standstill, the cooling is no longer available, i.e. the least amount of cooling is provided precisely when the most powerful flows of current are occurring. If there is no reaction to this state, for example by switching off the power, this can lead within seconds to a thermal failure of one or more components of the hand-guided power tool, for example a connecting line, the electric motor, or solder connections. Such a failure can also occur when the jamming occurs several times in succession without sufficient cool-down phases being observed between them. Typically in battery-powered hand-guided power tools, the thermal durability is increased through the use of high-temperature-resistant, low-ohm components, but this only delays the time at which the thermal failure eventually occurs. Alternatively, an analogous electrical switching-off can be used, which electrically interrupts the circuit when the current reaches a certain level. The components required for this, however, are expensive and can hamper the performance of the hand-guided power tool. The presetting of fixed current limits, however, also triggers a switching-off of the hand-guided power tool with the occurrence of temporary power surges, even though the tool would have been able to withstand them. Finally, there are known designs in which an NTC resistor measures the temperature of one or more critical components, for example the rechargeable battery pack, and when a predetermined maximum temperature is reached, the circuit is electrically interrupted. The disadvantage of this method, however, is that due to a low thermal conductivity or a volumetric expansion of the critical components, there can be a delay between the reaching of a critical temperature and the detection of this critical temperature.